1. Field of the Invention
The invention relates to heterologous gene expression. More particularly, the invention relates to the expression of malaria genes in higher eukaryote cell systems.
2. Summary of the Related Art
Recombinant production of certain heterologous gene products is often difficult in ill vitro cell culture systems or ill vivo recombinant production systems. For example, many researchers have found it difficult to express proteins derived from bacteria, parasites and virus in cell culture systems different from the cell from which the protein was originally derived, and particularly in mammalian cell culture systems. One example of a therapeutically important protein which has been difficult to produce by mammalian cells is the malaria merozoite surface protein (MSP-1).
Malaria is a serious heath problem in tropical countries. Resistance to existing drugs is fast developing and a vaccine is urgently needed. Of the number of antigens that get expressed during the life cycle of P. falciparum, MSP-1 is the most extensively studied and promises to be the most successful candidate for vaccination. Individuals exposed to P. falciparum develop antibodies against MSP-1, and studies have shown that there is a correlation between a naturally acquired immune response to NISP-1 and reduced malaria morbidity. In a number of studies, immunization with purified native MSP-1 or recombinant fragments of the protein has induced at least partial protection from the parasite (Diggs et al, (1993) Parasitol Today 9:300-302). Thus MSP-1 is an important target for the development of a vaccine against P. falciparum. 
MSP-1 is a 190-220 kDA glycoprotein. The C-terminal region has been the focus of recombinant production for use as a vaccine. However, a major problem in developing MSP-1 as a vaccine is the difficulty in obtaining recombinant proteins in bacterial or yeast expression systems that are equivalent in immunological potency to the affinity purified native protein (Chang et al., (1992) J. Immunol. 148:548-555.) and in large enough quantities to make vaccine production feasible.
Improved procedures for enhancing expression of sufficient quantities of MSP-1 would be advantageous.
The present invention provides improved recombinant DNA compositions and procedures for increasing the mRNA levels and protein expression of the malarial surface antigen MSP-1 in cell culture systems, mammalian cell culture systems, or in transgenic mammals. The preferred protein candidate for expression in an expression system in accordance with the invention is a C-terminal derivative of MSP-1 having a DNA coding sequence with reduced AT content, and eliminated mRNA instability motifs and rare codons relative to the recombinant expression systems. Thus, in a first aspect, the invention provides a DNA sequence derived from the sequence shown in SEQ ID NO 2. This derivative sequence is shown in SEQ ID NO 1.
In a second aspect, the invention provides a process for preparing a modified nucleic acid of the invention comprising the steps of lowering the overall AT content of the natural gene encoding MSP-1, eliminating all mRNA instability motifs and replacing all rare codons with a preferred codon of the mammary gland tissue, all by replacing specific codons in the natural gene with codons recognizable to, and preferably preferred by mammary gland tissue and which code for the same amino acids as the replaced codon. This aspect of the invention further includes modified nucleic acids prepared according to the process of the invention.
In a third aspect, the invention also provides vectors comprising modified MSP-1 nucleic acids of the invention and a goat beta casein promoter and signal sequence, and host cells transformed with nucleic acids of the invention.
In a fourth aspect, the invention provides transgenic non-human mammals whose germlines comprise a nucleic acid of the invention.
In a fifth aspect, the invention provides a DNA vaccine comprising a modified MSP-1 gene according to the invention.